A transmission-type display device is known which illuminates a transmissive display, such as an LCD (Liquid Crystal Display) panel, from the back by means of a surface source device (backlight) to display images.
Surface source devices are roughly classified into direct-light type devices which have a light source disposed right behind an optical member such as an optical sheet, and edge-light type devices which have a light source disposed lateral to an optical member. With a light source disposed lateral to an optical member such as a light guide plate, edge-light type surface source devices have the advantage that they can be designed to be thin as compared to direct-light type surface source devices, and have been widely used in recent years.
In an edge-light type surface source device, a light source is generally disposed in a position facing a light entrance surface which is a side surface of a light guide plate. Light emitted by the light source enters the light guide plate through the light entrance surface, and travels in the light guide plate from the light entrance surface toward the opposite surface in a direction (light guide direction) perpendicular to the light entrance surface while repeating reflection from a light exit surface and the opposite back surface.
The traveling direction of the light is changed e.g. by a diffusion pattern or prism shape, provided in the back surface of the light guide plate, so that the light will gradually exit the light guide plate at various positions on the light exit surface along the light guide direction and travel toward an LCD panel (see, for example, JP 09-43433A1).